SOP is a dual fertilizer containing 50% K2O and 18% S. It has the lowest salt index and is virtually free of chloride, which makes it a superior fertilizer to muriate of potash (MOP). On the other hand, MOP is easy to produce, especially, when brine/bittern is low in sulphate content such as in the Dead Sea and this accounts for its lower price compared to SOP. Countries such as India, which do not have low sulphate bittern, but which have adequate bittern of sea and sub-soil origin, would be greatly benefited if SOP can be produced economically from such bittern sources. Besides its application as a fertilizer, potassium sulphate has numerous industrial applications as well.
Mg(OH)2 is commercially used in pulp and paper industries and also as antacid and fire retardant. Waste water and acidic effluent treatment represent additional high growth areas for its application. Mg(OH)2 is also used for production of magnesia (MgO), magnesium carbonate and other magnesium chemicals. Mg(OH)2 that is low in B2O3 impurity is especially suitable for production of refractory grade MgO. High quality gypsum (CaSO4.2H2O) finds applications in the white cement industry and for manufacture of high strength α and β Plaster of Paris. Sodium chloride that contains small quantities of potassium chloride finds application in the edible salt industry.
Reference is made to the well-known Mannheim process involving reaction of MOP with sulphuric acid. The major problem with the process is that it is energy intensive and poses a problem of HCl management when no application of commensurate volume for HCl is available in the vicinity. J. A. Fernandez Lozano and A. Wint, (“Production of potassium sulphate by an ammoniation process”, Chemical Engineer, 349, pp 688–690, October 1979) disclose a process of SOP manufacture from MOP through reaction with gypsum in presence of ammonia. The principle of the process is double decomposition reaction between gypsum and potassium chloride in presence of ammonia at 0° C. The main disadvantage of the process is that it is energy intensive and necessitates careful design of the reactor for safe operation.
H. Scherzberg et al. (‘Messo pilots new potassium sulphate process’, Phosphorous & Potassium, 178, March–April 1992, p-20) describe the successful trials on a process involving reaction of MOP with sodium sulphate to produce the double salt glaserite (3K2SO4.Na2SO4). The glaserite is in turn reacted with MOP to produce SOP. The main disadvantage of the process is that it would be unsuitable for those who do not have access to such raw materials. Moreover, the process involves several complex unit operations including the need for chilling. Such processes have their limitation on large scale.
H. Scherzberg and R. Schmitz (‘Duisberg's alternative to Mannheim’, Phosphorous & Potassium, 178, March–April 1992, p-20), describe an integrated process for production of SOP from KCl and MgSO4 or Na2SO4. The main drawback of the process is that the amount of NaCl in raw materials has a critical effect on the process and, as such, is less applicable to crude mixed salt as obtained from sea bittern. Another disadvantage is that the process involves heating and cooling which makes it energy intensive. Yet another disadvantage is that the by-product obtained is MgCl2 in concentrated solution form which has a limited market and lower appeal compared to low B2O3 containing Mg(OH)2 solid produced as part of the integrated process of the present invention.
G. D. Bhatt et al. (‘Mixed Salt from Sea Bittern’, Salt Research & Industry, 2, 126–128, 1969) describe a process for the manufacture of mixed salt, i.e., comprising of a mixture of NaCl and kainite (KCl.MgSO43H2O), from sea bittern through solar evaporation and fractional crystallisation.
Patel et al. (Salt Research & Industry, Vol. 6, No. 14, 1969) disclose a process for the preparation of syngenite from mixed salt in pure form. K. P. Patel, R. P. Vyas and K. Seshadri (‘Potassium Sulphate from Syngenite’, Salt Research & Industry, Vol. 6, No. 2, April 1969) disclose a process for preparation of SOP by leaching syngenite (K2SO4.CaSO4.H2O) with hot water and then recovering it by solar evaporation. The main drawback of the process is that it is energy intensive. Moreover, production of syngenite from nixed salt is itself an involved affair.
K. Sehsadri et al (“Manufacture of Potassium chloride and byproducts from Sea Bittern” Salt Research and Industry, April–July 1970, Vol. 7, page 39–44) disclose a process wherein mixed salt (NaCl and kainite) obtained from bittern is dispersed with high density bittern in proper proportion and heated to a temperature of 110° C. when kieserite (MgSO4.H2O) is formed which is separated by filtering the slurry under hot conditions. The filtrate is cooled to ambient temperature, when carnallite crystallizes out. Carnallite is decomposed with water to get a solid mixture of sodium chloride and potassium chloride while magnesium chloride goes into solution. Solid mixture of potassium chloride and sodium chloride is purified using known techniques to produce pure potassium chloride. The drawbacks of this process are that it fails to make use of the sulphate content in bittern and, instead, offers an elaborate process for manufacture of MOP, which, in any case, is inferior to SOP as fertilizer.
U.S. Patent Application Number 2003/0080066 dated Oct. 29, 2001 by Vohra, Rajinder N. et. al. discloses an integrated process for recovery of high purity salt, potassium chloride, and end bittern containing 7.5 gpl Br. The process is based on desulphatation of brine with distiller waste of soda ash industry or calcium chloride generated from limestone and acid. The main drawback of the patent application is that the process is less attractive when distiller waste is not available in the vicinity and the process becomes less economical when carnallite has to be obtained from bittern without production of industrial grade salt. Moreover, as in the case referred to above, it is desirable to utilize the sulphate content in bittern and produce SOP in preference to MOP.
Michael Freeman (‘Great Salt Lake-A fertile harvest for IMC’ in Phosphorus & Potassium, 225, January–February, 2000) describe a process comprising concentrating the brine containing 0.2–0.4% KCl, harvesting mixed salt, separation of high sodium chloride fraction through floatation, leaching with sulphate rich brine to produce schoenite, hot water dissolution of schoenite, fractional crystallization of SOP and recycling of mother liquor containing up to 30% of original K to evaporation pond. The main drawbacks of the process are: (i) need for floatation which involves use of organic chemicals whose disposal is problematic, (ii) need for external heat for recovery of SOP from schoenite through fractional crystallization at elevated temperature, (iii) need for recycling of as much as 30% of K to evaporation ponds where it again gets contaminated with other components of the brine.
In Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, 1999, under the Chapter, Potassium compounds, a description of a process for production of SOP in Sicily is given, Kainite (KCl.MgSO4.2.75H2O), is obtained from a potash ore by flotation. It is then converted into schoenite at ca. 25° C. by stirring with mother liquor containing the sulfates of potassium and magnesium from the later stages of the process. Schoenite is filtered off and decomposed with water at ca. 48° C. This causes magnesium sulfate and part of the potassium sulfate to dissolve and most of the potassium sulfate to crystallize. The crystals are filtered and dried. The sulfate mother liquor is recycled to the kainite—schoenite conversion stage. The main drawbacks of the process are that there is no mention of the fate of the mother liquor obtained upon conversion of kainite into schoenite which would inevitably entail considerable loss of K, and the need for external source of heat to effect the fractional crystallization of SOP.
Chinese Patent No. 1281822 corresponding to application No. CN 2000-112497, 29 Aug. 2000, by Song, Wenyi; Liu, Yu; Zhao, Shixiang; Dai, Fangfa, titled method for preparing K2SO4 from sulphate type K-containing bittern. The method comprises concentrating the bittern, separating NaCl, concentrating to obtain crude K—Mg salt containing 10–45% NaCl, crushing, mixing with saturated bittern to obtain a solution with concentration of 20–40%, removing NaCl by back-floatation, concentrating, dewatering to obtain refined K—Mg salt containing less than 5% NaCl, mixing the K—Mg salt and water at specified ratio, allowing the mixture to react at 10–60° P for 0.5–3 hr, separating to obtain schoenite, mixing with KCl and water at specified ratio, allowing the mixture to react at 10–70° F. for 0.25–3 hr and separating to obtain K2SO4. The drawbacks of the process are (i) need for elaborate method of purification of mixed salt that includes removing NaCl by the less desirable method of back floatation that involves use of organic chemicals, (ii) lack of any mention of the manner in which the various effluent streams are dealt with, and (iii) dependence on outsourced KCl since no mention is made of any process for KCl production as part of the process.
J. H. Hildebrand (‘Extraction of Potash and other Constituents from sea water Bittern’ in Journal of Industrial and Engineering Chemistry, Vol. 10, No. 2, 1918, pp 96–106) describe theoretical aspects of the recovery of potash from sea bittern and propose a process for extraction. According to this process, bittern is evaporated at a temperature between 100–120° C., thereby forming a solid mixture of sodium chloride and kieserite (MgSO4.H2O), separating this mixture under hot conditions in a heated centrifuge, and cooling the mother liquor in a cooler for separation of carnallite. Carnallite is decomposed and washed with water to produce potassium chloride. The drawback of his process is that it is demanding in terms of energy requirement and sufficiently pure carnallite cannot be obtained. The main drawback of the process is the contamination of kieserite with NaCl, which would necessitate further purification to obtain products in saleable form. Another drawback of the process is that it requires energy to remove sulphate from bittern in the form of kieserite whereas it would be preferable to utilize the sulphate for the production of SOP.
D. J. Mehta et al (‘Production of Potassium Sulphate from Mixed Salt obtained from Salt Works of Little Rann Of Kutch’ Salt Research & Industry, Vol. 2 No. 4, October 1965) describe a process using floatation technique for the production of potassium sulphate from two types of mixed salt available from the salt works of the Little Rann of Kutch. The process suffers from the drawback of lack of suitability when high sulphate containing sea bittern is used and the need for froth floatation, which is costly, cumbersome and polluting.
Reference is made to the Chapter in Ullmamn's Encyclopedia of Industrial Chemistry, Sixth Edition, 2002, (Electronic Version) dealing with Magnesium Compounds written by Margarete Seeger, Walter Otto, Wilhelm Flich, Friedrich Bickelhaupt and Otto. S. Akkerman, wherein the process of preparation of magnesium hydroxide from seawater is described. It is mentioned therein that preparation of low boron containing magnesia requires over liming of the seawater up to pH 12 to maintain B2O3 content less than 0.05% in magnesia Over liming involves higher lime cost, need for neutralization of supernatant and results in a colloidal suspension which is not easy to filter. Another drawback is a lack of application of calcium chloride-containing effluent which is discharged back into be sea.
Patent Application No. 423211, CA 1203666, by Wendling et al titled, “Process for the manufacture of potassium sulphate by treatment of solution containing magnesium chloride and potassium chloride” describes a process for the production of potassium sulphate from solutions containing magnesium chloride, such as solutions of carnallite ore and, in particular, the equilibrium mother liquors of a unit for the treatment of carnallite. According to this process, sodium sulphate and potassium chloride are added to the solutions containing magnesium chloride, so as to precipitate sodium chloride and schoenite, K2SO4MgSO46H2O, and the schoenite obtained is treated in a known manner to produce potassium sulphate. The main drawback of the process is the need to outsource sodium sulphate and the lack of any mention of a solution to the problem of KCl loss in effluent streams.
H. Gurbuz et al. (‘Recovery of Potassium Salts from Bittern by Potassium Pentaborate Crystallisation’ in Separation Science & Technology, 31(6), 1996, pp. 857–870) disclose the preparation of sodium pentabotate from the reaction of Tincal and recycled H3BO3 in presence of water and thereafter treated with bittern to selectively precipitate out potassium pentaborate, which in turn is acidulated with sulphuric acid and fractionally crystallized to remove K2SO4 and recycle the H3BO3 in the process, The main drawbacks of the process are that the mother liquor contains significant quantities of boron, which entails elaborate procedure to recover boron and, moreover, the MgO obtained from such mother liquor would be unfit for industrial use. Moreover, although such a process can still be thought of for sulphate poor bittern, it would not be a preferred route when the bittern is rich in sulphate content. Yet another drawback is the need to chill the acidulated product for high yield.
A. S. Mehta (Indian Chemical Engineer, 45(2), 2003, p. 73) describes a process of bromine manufacture from bittern. Bittern is acidified with sulphuric acid to a pH of 3.0–3.5 and the bromide ion is then oxidized with chlorine and stripped off with the help of steam. The acidic de-brominated bittern is neutralized with lime, the sludge thus formed removed, and the effluent discharged. Bromine plants located in the vicinity of natural salt beds in the Greater Rann of Kutch in Gujarat, India utilize natural bittern for bromine production by the above method and discharge their effluent back into the Rann. Disposal of sludge poses a formidable challenge in these plants.
Chr. Balarew, D. Rabadjieva and S. Tepavitcharova (“Improved Treatment of Waste Brines” International Symposium on Salt 2000, page 531–554) describe recovery of marine chemicals. The authors describe the use of lime for precipitation of Mg(OH)2 from a part of available bittern, and desulphatation of balance bittern with the resultant CaCl2 solution for recovery of KCl via carnallite. The authors have not discussed any scheme of utilizing such methodology for production of SOP from sulphate-rich bittern. Moreover, as will be evident later, Mg(OH)2 produced directly from raw bittern has much higher B2O3 content compared to Mg(OH)2 prepared from the Mg2+ source of the present invention, which is linked to production of SOP.
Chinese Patent No. 1084492, Lu Zheng, describes a process of manufacture of SOP from bittern and potassium chloride. In this process, bittern is processed by evaporation, cooling, floatation, and is then reacted with potassium chloride to make potassium sulfate and by-products of industrial salt and residual brine. The main drawbacks of this process are that it requires involved separation techniques like floatation to remove NaCl from mixed salt and KCl required for production of SOP from schoenite has to be procured separately. Moreover, although overall yield in terms of potash recovery is 95%, yield with respect to such procured KCl is not mentioned.